The use of a separator between an anode and cathode in electrochemical cells is known. In the past, these separators have been generally porous separators, such as asbestos diaphragms, used to separate reacting chemicals within the cell. Particularly, for example, in diaphragm chlorine generating cells, such a separator functions to restrain back migration of OH.sup.- radicals from a cell compartment containing the cathode to a cell compartment containing the anode. A restriction upon OH.sup.- back migration has been found to decrease significantly overall electric current utilization inefficiencies in operation of the cells associated with a reaction of the OH.sup.- radical at the anode releasing oxygen.
More recently separators based upon an ion exchange copolymer have found increasing application in electrochemical cells. One copolymeric ion exchange material finding particular acceptance in electrochemical cells such as chlorine generation cells has been fluorocarbon vinyl ether copolymers known generally as perfluorocarbons and marketed by E. I. duPont under the name Nafion.TM..
These so-called perfluorocarbons are generally copolymers of two monomers with one monomer being selected from a group including vinyl fluoride, hexafluoropropylene, vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, perfluoro(alkylvinyl ether), tetrafluoroethylene and mixtures thereof.
The second monomer is selected from a group of monomers usually containing an SO.sub.2 F or sulfonyl fluoride group. Examples of such second monomers can be generically represented by the formula CF.sub.2 .dbd.CFR.sub.1 SO.sub.2 F. R.sub.1 in the generic formula is a bifunctional perfluorinated radical comprising 1 to 8 carbon atoms but occasionally as many as 25 carbon atoms. One restraint upon the generic formula is a general requirement for the presence of at least one fluorine atom on the carbon atom adjacent the --SO.sub.2 F, particularly where the functional group exists as the --(SO.sub.2 NH).sub.m Q form. In this form, Q can be hydrogen or an alkali or alkaline earth metal cation and m is the valence of Q. The R.sub.1 generic formula portion can be of any suitable or conventional configuration, but it has been found preferably that the vinyl radical comonomer join the R.sub.1 group through an ether linkage.
Typical sulfonyl fluoride containing monomers are set forth in U.S. Pat. Nos. 3,282,875; 3,041,317; 3,560,568; 3,718,627 and methods of preparation of intermediate perfluorocarbon copolymers are set forth in U.S. Pat. Nos. 3,041,317; 2,393,967; 2,559,752 and 2,593,583. These perfluorocarbons generally have pendant SO.sub.2 F based functional groups.
Chlorine cells equipped with separators fabricated from perfluorocarbon copolymers have been utilized to produce a somewhat concentrated caustic product containing quite low residual salt levels. Perfluorocarbon copolymers containing perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) comonomer have found particular acceptance in Cl.sub.2 cells.
In chlorine cells using a sodium chloride brine feedstock, one drawback of using perfluorocarbon separators having pendant sulfonyl flouride based functional groups has been a relatively low resistance in desirably thin separators to back migration of caustic including OH.sup.- radicals from the cathode to the anode compartment. This back migration contributes to a lower current utilization efficiency in operating the cell since the OH.sup.- radicals react at the anode to produce oxygen. Recently, it has been found that if pendant sulfonyl fluoride based cationic exchange groups adjacent one separator surface were converted to pendant carboxylate groups, the back migration of OH.sup.- radicals in such Cl.sub.2 cells would be significantly reduced. Conversion of sulfonyl fluoride groups to carboxylate groups is discussed in U.S. Pat. No. 4,151,053.
Presently, perfluorocarbon separators are generally fabricated by forming a thin membrane-like sheet under heat and pressure from one of the intermediate copolymers previously described. The ionic exchange capability of the copolymeric membrane is then activated by saponification with a suitable or conventional compound such as a strong caustic. Generally, such membranes are between 0.5 mil and 150 mil in thickness. Reinforced perfluorocarbon membranes have been fabricated, for example, as shown in U.S. Pat. No. 3,925,135.
Notwithstanding the use of such membrane separators, a remaining electrical power inefficiency in many electrochemical cells has been associated with a voltage drop between the cell anode and cathode attributable to passage of the electrical current through one or more electrolytes separating these electrodes remotely positioned on opposite sides of the cell separator.
Recent proposals have physically sandwiched a perfluorocarbon membrane between an anode-cathode pair. The membrane in such sandwich cell construction functions as an electrolyte between the anode-cathode pair, and the term solid polymer electrolyte (SPE) cell has come to be associated with such cells, the membrane being a solid polymer electrolyte. In some of these SPE proposals, one or more of the electrodes has been a composite of a fluoro resin polymer such as Teflon.TM., E. I. duPont polytetrafluoroethylene (PTFE), with a finely divided electrocatalytic anode material or a finely divided cathode material. In others, the SPE is sandwiched between two such polymeric electrodes. Typical sandwich SPE cells are described in U.S. Pat. Nos. 4,144,301; 4,057,479; 4,056,452 and 4,039,409. SPE composite electrode cells are described in U.S. Pat. Nos. 3,297,484; 4,212,174 and 4,214,958 and in Great Britain Patent Application Nos. 2,009,788A; 2,009,792A and 2,009,795A.
Use of the composite electrodes can significantly enhance cell electrical power efficiency. However, drawbacks associated with present composite electrode configurations have complicated realization of full efficiency benefits. Composite electrodes generally are formed from blends of particulate PTFE TEFLON and a metal particulate or particulate electrocatalytic compound. The PTFE blend is generally sintered into a decal-like patch that is then applied to a perfluorocarbon membrane. Heat and pressure are applied to the decal and membrane to obtain coadherence between them. A heating process generating heat sufficient to soften the PTFE for adherence to the sheet can present a risk of heat damage to cationic exchange properties of the membrane.
These PTFE TEFLON based composites demonstrate significant hydrophobic properties that can inhibit the rate of transfer of cell chemistry through the composite to and from the electrically active component of the composite. Therefore, TEFLON content of such electrodes must be limited. Formation of a porous composite has been proposed to ameliorate the generally hydrophobic nature of the PTFE composite electrodes, but simple porosity has not been sufficient to provide results potentially available when using a hydrophilic polymer in constructing the composite electrode.
To date efforts to utilize a hydrophilic polymer such as NAFION have been largely discouraged by difficulty in forming a commercially acceptable composite electrode utilizing NAFION. While presently composites are formed by sintering particles of PTFE TEFLON until the particles coadhere, it has been found that similar sintering of NAFION can significantly dilute the desirable cationic exchange performance characteristics of NAFION polymer in resulting composite electrodes.
An analogous difficulty has surfaced in the preparation of SPE sandwiches employing more conventional electrode structures. Generally these sandwich SPE electrode assemblies have been prepared by pressing a generally rectilinear electrode into one surface of a NAFION membrane. In some instances, a second similar electrode is simultaneously or subsequently pressed into the obverse membrane surface. To avoid heat damage to the NAFION membrane, considerable pressure, often as high as 6000 psi is required to embed the electrode firmly in the membrane. Depending upon the configuration of the embedded electrode material, such pressure is often required to be applied simultaneously over the entire electrode area, requiring a press of considerable proportions when preparing a commercial scale SPE electrode.
Often where a foraminous electrode such as a mesh of titanium coated with a chlorine release electrocatalyst or a nickel mesh contacts a membrane in a cell, gases released at the electrode adhere to portions of the membrane causing a blinding effect thereby restricting cation passage therethrough. This restriction elevates the electrical voltage required for cell operation, and thereby effectively increases operational power costs.
The use of alcohols to solvate particularly low equivalent weight perfluorocarbon copolymers is known. However, as yet, proposals for formation of perfluorocarbon composite electrodes and for solvent welding the composites to perfluorocarbon membranes where the perfluorocarbons are of relatively elevated equivalent weights desirable in, for example, chlorine cells, have not proven satisfactory. Dissatisfaction has been at least partly due to a lack of suitable techniques for dispersing or solvating in part these higher equivalent weight perfluorocarbons.
A number of patents assigned to Asahi Glass describe the concept of using particulate materials for imparting better gas release characteristics to the membrane. It is believed that the closest Asahi disclosure seen in prepublished Japanese Application No. 163,287/1981 which proposes that particles of "denatured" PTFE be included along with the metal oxide particulates rather than the ordinary PTFE previously proposed, (where "denatured" means that a minor amount of fluorocarbon monomer containing acidic functional groups has been combined with PTFE in forming said particles). However, this document still does not suggest that such "denatured" PTFE should or could be in solution when used to adhere the porous layer of solid oxides, etc. to an ion-exchange membrane, and, instead, makes use of auxiliary polymeric binders (such as carboxy methyl cellulose) in solution in water and/or alcohols. This invention distinguishes in that it comprises a method(s) to improve the adhesion of such gas release particles to the membrane as well as method(s) to apply a uniform coating to a fabric-reinforced membrane.